I too will echo the earlier answers. It is important that the this problem is solved correctly or the consequences (criminal prosecution) could be severe. I'll add that a possible option for your brother would be to seek the advice of an attorney who is familiar with the IRS' informal "voluntary disclosure" policy. While there are certain requirements (i.e. no IRS action encouraged your brother to come forward), if your brother qualifies, he may be able to limit his filing requirements to...

With respect to the question of the IRS coming after you for your ex husband's tax debt, unfortunately, the fact that he agreed to pay the tax in the marital settlement agreement will not be binding on the IRS. You may try responding to the IRS by providing them with a copy of the marital settlement agreement to see what they do. However, agreements between taxpayers have no effect on the application of the Internal Revenue Code.
You may consider filing an innocent spouse relief request...

Yes. Your marital status is determined as of the close of December 31 of any given year. If you are married at that time, you and your spouse (even if estranged) can file a joint tax return. While you could file "married filing separately" you cannot file as "single" if still legally married. If however, your divorce is final on December 30, you are considered single for that entire tax year and cannot file a joint tax return for the year.